Upgrade GitHub Action versions

[hyperupcall]

Description

Hi again! This fixes some errors from CI:

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, actions/upload-artifact@v4. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

This simply updates the actions to their latest major version.

Would you be willing to accept any of these follow-up PRs?:

• Harden GitHub Actions by using pinact and zizmor, for example pinning versions to a hash (ex. uses: anishathalye/proof-html@3cb0083534425b4902e6e77ac26ac3fe5b718f51 # v2.2.4, reducing impact of supply chain attacks similar to the ones we saw with tj-actions/changed-files.
• Use GitHub Actions to build Jekyll website instead of relying on internal environment. This satisfies the other build warning in CI: The github-pages gem can't satisfy your Gemfile's dependencies. If you want to use a different Jekyll version or need additional dependencies, consider building Jekyll site with GitHub Actions: https://jekyllrb.com/docs/continuous-integration/github-actions/.

Copyright Assignment

• This document is covered by the MIT License, and I agree to contribute this PR under the terms of the license.

[unixorn]

lgtm

[unixorn]

Yes, I will accept all the help I can get 😄

I also just noticed I never added a dependabot config to this repo